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Lecture Note Chapter 3 & 4.pptx
1. Chapter 3
3. Environmental Sampling: Purpose, Design Strategy and
Techniques
A sample is “a smaller (but hopefully representative) collection of units from an
environment used to determine truths about that environment”.
The sample is the source of information about the environment.
Sampling is the act of collecting a portion of material for analytical purposes that
accurately represents the material being sampled with respect to stated objectives.
In other words, the primary aim of the sampling is to produce a set of samples
that accurately represent the source of interest (the environment) and suitable for
analysis.
Real materials are usually not homogeneous, so the sample must be chosen carefully
to be representative of the real material.
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2. Representative sample
• It is one that reflects the true value and distribution of the analyte in the original
material.
• the central concern is to avoid biases in the information will be generated.
• if the sample is not representative, no matter how excellent the analytical method
or how expert the analyst, the analysis will be not only a waste of time, but can
also lead to erroneous conclusions
Sampling Design and Strategy
• The main objective in any sampling strategy is to obtain a representative portion of
the sample
• planning of the sampling strategy is an important part of the overall analytical
procedure ---
» to reduce time, cost , money and also to get correct answer
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3. To develop sampling plan four Primary factors are needed:
1. Objectives -determining factor in sampling design
2. Environmental variability -If an environmental domain is completely
homogeneous, a single sample would adequately represent it. However, we
seldom come across such a situation, as the environment is highly heterogeneous.
– One must also distinguish between a static and a dynamic system.
3. Expenses - sampling and analysis cost should certainly be a consideration in all
environmental study.
– The cost-effectiveness of a sampling design should be evaluated in the design
stage so that the chosen sampling design will achieve a specified level of data
quality at a minimum cost or, an acceptable level of data quality at a prespecified
cost
4. Nontechnical factors: such as sampling convenience, site accessibility,
availability of sampling equipment, and political aspects.
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4. Where (Space) and When (Time) to Sample
• Environmental sampling can be viewed in a space-time domain.
• In the time domain, there is only one dimension (1-D).
• Specific time can be designated as t1, t2, t3, . . .tN in a time period such as days, weeks,
months, or years.
• In the space domain, analyte variations and hence sampling points can be designated
by the coordinates in 1-D (x), 2-D (x, y), or 3-D (x, y, z).
• An example of 1-D problem is an outfall of an industrial wastewater discharge.
• We would only be concerned about the concentration as a function of the
downstream distance from the discharge point.
• An example of 2-D problem is the radiochemical contents in surface soil due to
atmospheric deposition from a nuclear weapon testing site or the lead content in
surface soils downwind from a local smelter facility.
• In these two cases, the soil depth may not be the primary interest, so soil samples
may be designated by longitude and latitude, or by measurements relative to an
existing structure.
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5. • A 3-D sampling site is common in a large body of water, or a solid/hazardous waste
landfill site where depth is a variable for contaminant variations.
How many samples should be collected?
• Unless the material being sampled is known to be well mixed, it is unlikely that a
single measure will be representative of the source body of material.
• If the target population is homogeneous, then we can collect individual samples
without giving consideration to where to sample. Unfortunately, in most situations
the target population is heterogeneous
• A sampling error occurs whenever a sample’s composition is not identical to its
target population.
• The best sample number is the largest sample number possible.--- but it is
sometimes impossible
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6. • A minimum is three data points per site for basic statistical tests, but more may
be required depending on the inherent variability in the measurement data.
• The number of data points needed may not be known until after chemical analysis of
some samples.
• It is good practice to take additional samples and to store these for subsequent
analysis if required, although short maximum holding times for some contaminants
may mean this is impractical.
Sample Amounts
• The minimally required sample amount depends primarily on the concentration of
the analytes present in sample matrices.
• The sample amount should be sufficient to perform all required laboratory
analyses with an additional amount remaining to provide for analysis of quality
assurance/quality control samples including duplicates and spikes.
• Another determining factor is the representativeness associated with the sample
amount. 6
7. • For heterogeneous samples, a larger portion is generally required to be a
representative of the actual sample variations such that possible biased results can be
minimized.
• This larger portion of the collected sample is then homogenized thoroughly followed
by sub-sampling.
• Taking too large or too many samples should be avoided because storage (cold room
and freezer), transportation, and disposal costs can become a burden to a big project.
Water/wastewater sample Amounts
• Minimal liquid sample volume varies considerably in the range of 5 mL for Total
petroleum hydrocarbons (TPHs) in liquid wastes, 100 mL for total metals, and 1
L for trace organics such as pesticides.
• This bulk estimate of sample size represents a volume sufficient to perform one
analysis only, and as a general guide, the minimum volume collected should be three
to four times the amount required for the analysis
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8. Soil/sediment/solid waste Amounts
• A full characterization of soil physicochemical properties including particle size and
soil texture, as required for most environmental projects, usually needs a minimum of
approximately 200 g of soil.
• For most of the contaminant analysis, however, a dry mass of approximately 5–100 g
is sufficient.
• More soil samples are needed if the goal is to detect low solubility (hydrophobic)
organic contaminants in the aquifer materials since these chemicals tend to
accumulate in the first six inches of surface soils.
• The required sample size for the sediment is smaller than the water samples because
contaminants tend to accumulate in the sediment as a sink.
• Because waste samples are generally of high concentrations, sample volumes are of
less concern, but the volume should be kept at a minimum to reduce disposal costs.
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9. Water/sediment samples amounts for toxicity testing
• Unlike chemical analysis, the water and sediment used as a substrate for toxicity
testing demand more samples.
• For instance, 20–40 L of water is needed to perform an effluent acute toxicity test.
For the sediment, 15 L is required to conduct bioaccumulation tests (based on an
average of 3 L sediment per test chamber and 5 replicates), and 8–16 L sediment is
needed to conduct benthic macrointe vertebrate assessments.
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10. Sampling Techniques for Air, Water, Soil, and Biological
Materials
Sampling Methods
• There are several approaches to sampling: judgmental (nonstatistical), random,
stratified and systematic.
• More than one of these may be applied at the same time.
• A statistical approach is taken to increase the accuracy and decrease bias.
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11. 1. Judgmental Sampling
• It refers to the subjective selection of sampling locations based on professional
judgment using prior information on the sampling site, visual inspection (e.g.,
leaks and discoloration), and/or personal knowledge and experience.
• It is the preferred sampling approach when schedule (such as for emergency spill
response) and budget are tight.
• Judgment sampling is also preferred at the early stage of site investigation or when
the study objective is to just screen an area for the presence or absence of
contamination so a prompt decision can be made whether or not a follow-up
statistical sampling is needed.
• It is a non-statistical sampling procedure.
• Judgmental sampling is more biased than random sampling, but requires fewer
samples
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12. 2. Random Sampling
• sampling locations are chosen randomly.
• It is most useful when the pollutant of interest is relatively homogeneous in
the sampling medium
• In random sampling the sub-samples are chosen randomly from any location
within the material being tested.
• It is often preferred because it avoids human bias in selecting samples and
because it facilitates the application of statistics.
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13. • Limitations
– not applicable for heterogeneous populations…this is b/ce
• sampling points by random chance may not be uniformly dispersed in space
and/or time particularly when sample size is small. This drawback can be
overcome by systematic sampling.
• simple random sampling, by ignoring prior site information or professional
knowledge, often leads to more samples, which is not as cost-effective as
other sampling designs.
• randomly selected sampling points could be harder to locate precisely.
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14. 3. Stratified Sampling
• It is commonly used in a location which is known to have contaminants
heterogeneously distributed.
• the site is sub-divided into smaller areas (strata) and within each stratum a simple
random sampling is employed.
• The sub-dividing of the site can be carried out either to give equal areas, or be related
to known features within the site
• The selection of strata, however, requires some prior knowledge of the area to be
sampled.
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15. • ‘‘Strata’’ could be
– ‘‘Temporal strata ’’ -- permit different samples to be selected for specified time
periods, for example, day vs. evening, weekdays vs. weekend, four seasons of the
year, and so forth.
– “Spatial strata” -- are more common and come with many varieties. It can be
based on sampling depth (stratified lakes, soil, or sediment cores), ages and sex
of population (men, women and children), topography, geographical regions,
land types and uses, zones of contamination, wind direction (downwind vs.
upwind), political boundaries, and so forth.
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16. 4. Systematic Sampling
• Systematic sampling involves selecting sample units according to a specified pattern
in time or space, for example, at equal distance intervals along a line or a grid pattern
• Some slight variations exist for systematic sampling as shown in Figure for a two-
dimensional space sampling, (Fig. a), and (Fig. b).
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a) systematic grid sampling b) systematic random sampling
17. Types of Samples
• There are three common methods for obtaining samples: grab sampling, composite
sampling, and in situ sampling.
1. Grab sampling
• we collect a portion of the target population at a specific time and/or location,
providing a “snapshot” of the target population.
• If our target population is homogeneous, a series of random grab samples allows
us to establish its properties.
• For a heterogeneous target population, systematic grab sampling allows us to
characterize how its properties change over time and/or space
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18. 2. Composite sampling
• is a set of grab samples that we combine into a single sample before analysis.
• Grab samples are collected at random sampling points and mixed together into a
composite sample prior to analysis.
• Because information is lost when we combine individual samples, we normally
analyze grab sample separately.
• The advantage of this type of sampling is that a greater number of samples may be
collected to better represent the mean contaminant concentration of the sampled
medium without an increase in the analytical cost.
• A common practice, for example, in clinical laboratories screening samples for drug
abuse among athletes is to analyze a composite of about 10 samples. If the composite
produces a positive result, then the individual samples are tested.
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19. Sampling a flowing system, with compositing. One liter samples are collected at a
random time each hour. These are composited into two samples which are then
subdivided for analysis. This scheme assumes that the flow is constant 19
20. 3. In situ sampling
• A significant disadvantage of grab samples and composite samples is that we cannot
use them to continuously monitor a time-dependent change in the target population.
• In situ sampling, in which we insert an analytical sensor into the target population,
allows us to continuously monitor the target population without removing individual
grab samples.
• For example, we can monitor the pH of a solution moving through an industrial
production line by immersing a pH electrode in the solution’s flow.
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21. Water Sampling
• Water samples can come from many sources: ground water from wells, precipitation
(rain or snow), surface water (lakes, river, etc), ice or glacial melt, wastewater
(domestic, landfill, mine runoff, etc.), industrial process water and drinking water.
• Contaminants are distributed in the aqueous phase and in the particles suspended in
the water.
• Insoluble solids and liquids with densities less than water (such as oils and grease)
tend to float on the surface, while those with higher density sink to the bottom.
• The composition of stagnant water varies with the seasons and also with ambient
temperatures.
• Many different types of manual and automatic samplers are commercially
available.
• They are designed to collect grab samples or composite samples.
• Particular attention is given to material of construction of the sampler. Stainless steel
or Teflon is preferred because of their inert 21
22. i. Surface Water Sampling
• Sampling surface water sources such as lakes, flowing rivers and streams can be
quite challenging.
• Shallow depths can be sampled as easily as by dipping a container and collecting
water. However, sampling at depth in stratified sources can offer unique
challenges.
• Prior to sampling, surface water drainage around the sampling site should be
characterized.
• In a flowing water stream, sampling should be carried out downstream before
sampling upstream, because the disturbance caused by upstream sampling activity
may affect water quality downstream. Similarly, if water and sediment samples are
to be collected at the same point, the water sample should be collected before the
sediment disturbed.
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23. • The simplest sampling device is a dipper (or a container) made of stainless steel or
Teflon. The device is filled by slowly submerging the sampler into the water with
minimum disturbance and the water is transferred to the sample bottle.
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24. • Most of them work on the general principle that a weighted bottle is lowered to the
specified depth. At this point, a stopper or cap is opened remotely and the bottle is
allowed to fill. Then the stopper is closed to prevent any water from flowing in or out
and the bottle is pulled out.
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Weighted bottle sampler
25. ii. Ground Water Well Sampling
• To obtain ground water samples, monitoring wells, from which water samples can be
collected, are drilled into the ground.
• Care should be taken so that the water does not get contaminated during the drilling
process or contaminants do enter the water from the surface through the well.
• To ensure that the sample represents the water in the well and contaminants from
drilling are not present in the sample, some water is removed from the well before a
sample is collected.
• The amount of water purged depends upon the diameter, depth, and the refill rate of
the well. The purge amount is usually 3–10 times the well volume. In some cases, the
pH, conductance, or temperature is monitored until a constant value is reached. Then
a sample is collected.
• Various bailers and pumps are used in ground water sampling. Bailers are made of
stainless steel or Teflon with a check valve at the bottom. The check valve opens to
fill the sample, but closes when the sample is brought up. 25
26. • Groundwater bailer: sampler fills automatically, and the check valve in the bottom
keeps the sample from flowing out as the bailer is retrieved from the well.
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27. Biological Tissue Sampling
• Contaminants in water or soil often find their way into the food chain and
bioaccumulate in plant or animal tissues.
• The sampling and analysis of various specimens from the biota may be a good way to
establish the extent of contamination. Often more pertinent information on the extent
of damage done by a particular contaminant can be found in this way, rather than by
analyses of water and sediment.
• As an example, the extent of water contamination is often determined by a study of
fish or other aquatic organisms.
• This is not only useful for checking on water quality, but also provides a guide to the
acceptability of the fish for consumption.
• Fish tissues are often analyzed for metals and for organic pollutants such as PCBs,
and pesticides
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28. Soil Sampling
• Soil is quite heterogeneous containing rocks, trapped gases, and liquids.
• It varies across the surface, and with depth.
• This variation is caused by contact with the atmosphere and the biosphere, as well as by
the flow of ground water.
• Soil sampling devices must be made of tough material which can be forced into the soil.
• These are usually brass, steel, or plastic, sometimes Teflon coated to prevent
contamination of the samples by the metals used in construction of the sampler.
Stainless-steel sampling devices are most popular.
• Soil samples collected from the uppermost foot of the soil can be obtained using a
sample scoop.
• For obtaining samples from a greater depth, a device that can drill into the ground has to
be used.
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29. Scoop
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Tube corer
Auger and tube sampler. The auger is used to
form a hole down to the desired depth. The tube
is then driven into the soil to bring up a core
sample.
31. Chapter 4
Sample Preservation and Preparation for Environmental
Analysis
Preservation of Environmental Samples
• Sampling ……. > preservation.
• most laboratories cannot analyse samples immediately upon receipt, so some form
of sample storage is almost always required, but:
• losses can occur
• or potential contaminants can enter the sample
These problems can all lead to the analyst getting the wrong answer
or at least an unexpected answer after the analysis has taken place.
• Sample preservation is the measure or measures taken to prevent reduction or loss
of target analytes.
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32. • Analyte loss can occur between sample collection and laboratory analysis because of
• Physical processes that may degrade a sample are volatilization, diffusion, and
adsorption on surfaces.,
• Chemical processes, include photochemical reactions, oxidation, and precipitation
• Biological processes include biodegradation and enzymatic reactions.
• The purpose of sample preservation is to minimize any physical, chemical, and/or
biological changes that may take place in a sample from the time of sample collection
to the time of sample analysis.
• Three approaches generally used to minimize such change :
• refrigeration,
• use of proper sample container
• addition of preserving chemicals
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33. 33
Methods of sample preservation to minimize potential changes of analytes
during sample transportation and storage
34. • Refrigeration does not help to preserve acidified water samples for metal analysis.
Cold storage will adversely reduce metal solubility and enhance precipitation in the
solution.
• Even with the proper preservation, no samples can be stored for an extended period
of time without significant degradation of the analyte.
• The maximum holding times (MHTs) are the lengths of time a sample can be stored
after collection and prior to analysis
• The holding time depends on the analyte of interest and the sample matrix.
• For example, most dissolved metals are stable for months, whereas Cr(VI) is stable
for only 24 hours.
• In addition, MHTs vary with sample matrix, and analytical methodology used to
quantify the analyte’s concentration.
• Several parameters must be measured immediately in the field, such as pH,
temperature, salinity, and DO. Many other parameters must be measured within 1–2
days after sample collection.
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35. • Many of the organic compounds (purgeable hydrocarbons, pesticides, and
base/neutral/acid extractable organics) have only 7 days of permissible storage time
until sample pretreatment.
• Only samples for hardness and general metals can be stored for up to 6 months
after the addition of HNO3 to pH < 2.
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36. Absorption of Gases from the Atmosphere
• Gases from the atmosphere can be absorbed by the sample during handling, for
example, when liquids are being poured into containers.
• Gases such as O2, CO2, and volatile organics may dissolve in the samples. Oxygen
may oxidize species, such as sulfite or sulfide to sulfate.
• Absorption of CO2 may change conductance or pH.
• This is why pH measurements are always made at the site.
• CO2 can also bring about precipitation of some metals.
• Dissolution of organics may lead to false positives for compounds that were actually
absent.
• Blanks are used to check for contamination during sampling, transport, and
laboratory handling.
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37. Chemical Changes and biological processes
• For inorganic samples, controlling the pH can be useful in preventing chemical
reactions.
• For example, metal ions may oxidize to form insoluble oxides or
hydroxides.
• The sample is often acidified with HNO3 to a pH below 2, as most
nitrates are soluble, and excess nitrate prevents precipitation.
• Other ions, such as sulfides and cyanides, are also preserved by pH
control.
• Samples collected for NH3 analysis are acidified with sulfuric acid to
stabilize the NH3 as NH4SO4.
• Organic species can also undergo changes due to chemical reactions.
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38. • Storing the sample in amber bottles prevents photooxidation of organics (e.g.,
polynuclear aromatic hydrocarbons).
• Organics can also react with dissolved gases;
• for example, organics can react with trace chlorine to form halogenated
compounds in treated drinking water samples.
• In this case, the addition of sodium thiosulfate can remove the chlorine.
• Samples may also contain microorganisms, which may degrade the sample
biologically. Extreme pH (high or low) and low temperature can minimize microbial
degradation. Adding biocides such as mercuric chloride or pentachlorophenol can
also kill the microbes.
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39. Selection of Sample Containers
• Factors to be considered are costs, ease of use, and cleanliness.
• containers should be compatible to the analytes in a particular matrix.
• For water samples, containers are selected as follows:
1) Glass vs. plastics
• Glass containers may leach certain amount of boron and silica, and significant
sorption of metal ions may take place on the container wall.
• Glass containers are generally used for organic compounds and plastic
containers are used for inorganic metals.
• For trace organics, the cap and liner should be made of inert materials so that
sorption and diffusion will not be a potential problem.
• Plastic containers can be used for physical properties, inorganic minerals, and
metals.
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40. 2) Headspace vs. no headspace
• No headspace is allowed for the storage of samples used for the analysis of volatile
organic compounds (VOCs). Even a very small bubble will invalidate the analytical
results.
• Do not overfill the sample container and do not subdivide the sample in the
laboratory for oil and grease analysis.
3) Special containers
• Use specific container such as DO/BOD bottles and VOC vials.
• The BOD bottle is a narrow-mouth glass stopped container with 300-mL capacity,
which has a tapered and pointed ground-glass stopper and flared mouth.
Common steps in sample preservation are the use of proper containers,
temperature control, addition of preservatives, and the observance of
recommended sample holding time. 40
41. Sample Preparation for Environmental Analysis
• The sample preparation step in an analytical process typically consists of an
extraction procedure that results in the isolation and enrichment of components of
interest from a sample matrix.
• several processes
• crushing (size reduction), homogenization, dissolution, chemical digestion
with acid or alkali, sample extraction, sample cleanup and sample pre-
concentration.
• More operations require longer processing time and may lead to more error
sources and possibly less accurate analytical results.
• However, in many cases extensive sample preparation is applied for better results.
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42. Sample preparation for Metal Analysis
• Many metal analyses are carried out using atomic spectroscopic methods such as
flame or graphite furnace atomic absorption or inductively coupled plasma atomic
emission spectroscopy (ICP-AES).
• These methods commonly require the sample to be presented as a dilute aqueous
solution, usually in acid.
• Sample preparations for total metal analysis are always performed by one of the two
common acid digestion procedures, that is,
– the classical hotplate digestion method and
– the microwave-assisted acid digestion method
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43. Wet Ashing
1) Hotplate Acid Digestion
• the purpose of acid digestion is to dissolve metals from sample matrix so that
metals can be in a measurable form.
• In the case of organic matrices, an oxidizing mixture is used to destroy the entire
organic matrix and solubilize the sample. This yields a clear solution containing
the metals for analysis.
Various acids are used in acid digestion
• The choice of an individual acid or combination of acids is dependent upon the
nature of the matrix to be decomposed and what instrument is available for metal
analysis.
• Nitric acid (HNO3) is commonly used. There are several reasons why HNO3 is
preferred over other acids.
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44. • There is no chance of forming insoluble salts as might happen with HCl or H2SO4.
• HNO3 is acting as an oxidizing agent in the digestion process.
• HCl is not preferred if graphite furnace atomic absorption spectroscopy (GFAA) is
the method of analysis because of the interference from chloride
• The second factor relates to the characteristics of the sample matrix.
• HNO3 is an acid of preferred choice either alone or in combination with other
acid(s).
• The only exception to use HNO3 is when samples contain highly concentrated
alcohols and aromatic rings that can form explosive compounds (e.g., nitro glycerine
and TNT).
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45. A summary of the most common acids types used and their
applications
• For clean samples or easily oxidized materials, the use of HNO3 alone is
adequate,
• For readily oxidizable organic matter, the HNO3–HCl or HNO3–H2SO4
digestion is adequate,
• For difficult to-oxidize organic matter, HNO3-HClO4 is needed, and
• If matrix contains silicates, then HNO3–HClO4–HF digestion is necessary.
• No acids other than HF will liberate the metal of interest from the silica
matrix.
Hydrogen peroxide (H2O2) may be added to increase the oxidizing power of
the digestion. 45
46. Acid Digestion Procedure
the sample is placed into an appropriate vessel for the decomposition stage. The
simplest method for wet digestion is carried out in an open container.
Samples are dried, weighed, and placed in a beaker.
The digestion reagent is added.
The beaker is covered with a watch glass and placed on a hot plate,
The sample is allowed to boil very gently to avoid spattering.
More solution may be added from time to time to prevent the sample from drying
out.
Hydrogen peroxide may be added at a point during the digestion to help oxidize
organic materials.
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47. • When the sample has been digested completely, it is evaporated to near dryness and
then taken up in a dilute acid solution and diluted to volume for analysis.
• Samples are generally not allowed to dry completely, as species even less soluble
may form.
• Filtration at this point is often necessary, as many matrices will leave some insoluble
matter, such as silica.
• The filter must be rinsed carefully to avoid the loss of analyte.
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Open digestion can be done on a hotplate
in a loosely covered beaker
49. 2) Microwave Digestion
• The first reported use of a microwave oven for the acid digestion of samples for
metal analysis was in 1975.
• Advances in technology by a variety of manufacturers mean that today there are two
types of microwave heating systems commercially available, i.e. an open-focused
and a closed-vessel system (for background information on microwave heating).
• In the open-style system, up to six sample vessels are heated simultaneously.
• The sample and acid (sulfuric acid can be used) are introduced into a glass container,
which has the appearance of a large boiling/test tube, and is then fitted with a
condenser to prevent loss of volatiles.
• The sample container is placed within the microwave cavity and heated.
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51. Dry Ashing
• For samples that contain much organic matter, which are being analyzed for
nonvolatile metals, dry ashing is a relatively simple method of removing the
organic matter that can be used for relatively large samples and requires little of the
analyst’s time.
• In the open vessel method, the sample is placed in a suitable crucible and is ignited in
a muffle furnace in the presence of air at 400–800◦C. Crucibles used for ashing are
usually made of silica, porcelain, platinum, or Pyrex glass.
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52. General procedure
Place the weighed sample into a platinum or silica glass crucible and heat it in a
muffle furnace to a white ash.
After decomposition, the residue is transferred to a volumetric flask and dissolved in
concentrated nitric acid and warm water, and then diluted to volume prior to analysis.
This allows organic matter to be destroyed.
However, the method may also lead to the loss of volatile elements, e.g. Hg, Pb, Cd,
Ca, As, Sb, Cr and Cu.
Thus, while compounds can be added to retard the loss of volatiles, its use is
limited.
Salts or sulfuric acid may be added, if needed, and a final ashing step can be done
with hydrofluoric acid if required.
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53. • The carbon present in the sample oxidizes to CO2, and hydrogen, sulfur,
and nitrogen leave as H2O, SO2, and N2.
• These gases can be trapped and weighed to determine their concentration
in the organic material.
• Often the goal of dry ashing is to remove the organic material, leaving
behind an inorganic residue, or ash, that can be further analyzed.
• Due to the disadvantages of this method, namely:
– losses due to volatilization
– resistance to ashing by some materials
– difficult dissolution of ashed materials high risk of contamination
– It has largely been replaced by wet ashing.
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54. Sample Preparation for Organic Analysis
A) For Solid Samples
• extraction of environmental pollutants from solid or semi-solid matrices can be
divided into several categories based on the method of extraction, mode of heating
and presence or not of some type of agitation.
1. Soxhlet Extraction
• used as the benchmark against which any new extraction technique is compared. The
basic Soxhlet extraction apparatus consists of a solvent reservoir, an extraction body,
a heat source (e.g. an isomantle) and a water-cooled reflux condenser
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55. • A Soxhlet uses a range of organic solvents to remove organic compounds, primarily
from solid matrices.
– The solid sample (ca. 10 g if a soil) and a similar mass of anhydrous sodium
sulfate are placed in the porous thimble (cellulose), which in turn is located in the
inner tube of the Soxhlet apparatus.
– The apparatus is then fitted to a round-bottomed flask of appropriate volume
containing the organic solvent of choice, and to a reflux condenser.
– The solvent is then boiled gently using an isomantle – the solvent vapour passes
up through the tube marked (A), is condensed by the reflux condenser, and the
condensed solvent falls into the thimble and slowly fills the body of the Soxhlet
apparatus.
– When the solvent reaches the top of the tube (B), it syphons over into the round-
bottomed flask the organic solvent containing the analyte extracted from the
sample in the thimble. The solvent is then said to have completed one cycle.
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56. The whole process is repeated frequently until the pre-set extraction time is reached.
As the extracted analyte will normally have a higher boiling point than the solvent,
it is preferentially retained in the flask and fresh solvent recirculates.
This ensures that only fresh solvent is used to extract the analyte from the sample in
the thimble.
A disadvantage of this approach is that the organic solvent is below its boiling
point when it passes through the sample contained in the thimble.
In practice, this is not necessarily a problem as Soxhlet extraction is normally carried
out over long time-periods, i.e. 6, 12, 18 or 24 h.
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59. 2. Shake-Flask Extraction (Maceration)
• Conventional liquid–solid extraction, in the form of shake-flask extraction, is carried
out by placing a soil sample into a suitable glass container, adding a suitable organic
solvent, and then agitating or shaking.
• Agitating or shaking is carried out for a pre-specified time-period.
• After extraction, the solvent containing the analyte needs to be separated from the
matrix by means of centrifugation and/or filtration.
• In some instances, it may be advisable to repeat the process several times with fresh
solvent and then combine all of the extracts.
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61. 3. Digestion
• This is a form of maceration in which gentle heat (40-60oC) is applied during the
process of extraction.
• It is used when moderately elevated temperature is not objectionable.
• The process may be modified by mixing the material with the solvent using magnetic
stirrer, mechanical stirrer or by shaking occasionally by hand.
• After 8 to12 hours, the extract is filtered and fresh solvent is added and the process
repeated till all the desired solutes are extracted.
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62. B) For liquid (Aqueous) Samples
• There are several solvent extraction techniques for the analysis of non- and
semivolatile organic compounds in a liquid state.
• Separatory funnel liquid–liquid extraction, continuous liquid–liquid extraction,
and solid-phase extraction (SPE) techniques most often are used for liquid matrices.
• The solvents used for liquid–liquid extraction techniques are insoluble in the aqueous
sample. The techniques are applicable for the extraction of water-insoluble and
slightly water-soluble organic compounds.
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63. 1) Separator funnel liquid–liquid extraction
• This technique is a classic approach to extraction for liquid samples for a spectrum of
non- and semi volatile organic compounds.
• An aqueous sample is mixed in a separatory funnel with an immiscible organic
solvent that is denser than water.
• After standing, the mixture will separate into two phases with the analytes
partitioning toward the organic phase.
• The solvent is drawn off and saved, and the extraction step is repeated multiple
times.
• The solvent extracts are combined for the analytical step.
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65. 2) Continuous liquid–liquid extraction
• This technique is an automated version of the separatory funnel technique for a
spectrum of non- and semivolatile organic compounds.
• The solvent is added to the top of a liquid–liquid extractor, which contains the
aqueous sample.
• The solvent extracts the analytes as it passes through the sample.
• The extract is collected in a boiling flask and distilled, and fresh solvent is sent to the
top of the extractor to create a continuous process.
• This process runs for 12–24 h, and it is used in situations in which large sample sizes
with low analyte concentrations are needed.
• The extract contained in the boiling flask is used for the analytical step.
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66. 66
• the construction principles of two types of continuous liquid–liquid
extraction systems.
(a)less dense and
(b) more dense than the solution from which the solute is being extracted.
67. 3. Solid-phase Extraction (SPE) Techniques
• SPE is a sample preparation technology that uses solid particle,
chromatographic packing material, usually contained in a
cartridge type device, to chemically separate the different
components of a sample.
• Samples are nearly always in the liquid state
• SPE uses the difference of affinity between an analyte and
interferents, present in a liquid matrix, for a solid phase (sorbent).
• This affinity allows the separation of the target analyte from the
interferents.
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68. • A typical solid phase extraction involves four steps:
1. First, the cartridge is equilibrated or conditioned with a solvent
to wet the sorbent.
2. Then the loading solution containing the analyte is percolated
through the solid phase. Ideally, the analyte and some impurities
are retained on the sorbent.
3. The sorbent is then washed to remove impurities.
4. The analyte is collected during this elution step.
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